1. Field of the Invention
The invention relates to a fuel cell system and a control method for a fuel cell system.
2. Description of Related Art
A fuel cell that generates electric power by the electrochemical reaction between fuel gas and oxidant gas attracts attention as an energy source. The fuel cell includes a polymer electrolyte fuel cell that uses a polymer electrolyte membrane as an electrolyte membrane. The polymer electrolyte fuel cell generally uses a membrane electrode assembly in which an anode and a cathode are respectively bonded to both surfaces of the electrolyte membrane. Then, in the polymer electrolyte fuel cell, in order to obtain desired power generation performance, it is necessary to keep the electrolyte membrane in an appropriate wet state to thereby appropriately maintain the proton conductivity of the electrolyte membrane.
In such a polymer electrolyte fuel cell, during power generation, there occurs a nonuniform water content distribution (nonuniform distribution of wet state) within the plane of the electrolyte membrane of the membrane electrode assembly, and the nonuniform water content distribution may cause a nonuniform power generation distribution. Then, when there occurs a locally insufficient water content within the plane of the electrolyte membrane, the amount of power generation per unit area may exceed an allowable value in another region where there is no insufficient water content. Hereinafter, in the membrane electrode assembly, the fact that the amount of power generation per unit area locally exceeds an allowable value is termed “power generation locally concentrates” or “local power generation concentration”. Then, the power generation concentration leads to local degradation of the membrane electrode assembly. In addition, in an oxidant gas flow passage for flowing oxidant gas along the surface of the cathode, for example, there may occur a nonuniform power generation distribution caused by a nonuniform distribution of residual water that is produced during power generation and remains as liquid. Then, when there is a locally excessive amount of residual water in the oxidant gas flow passage, oxidant gas supplied to part of region of the cathode becomes insufficient, so power generation locally concentrates in another region where oxidant gas supplied is not insufficient to thereby lead to local degradation of the membrane electrode assembly. This also applies to a fuel gas flow passage for flowing fuel gas along the surface of the anode. That is, local power generation concentration due to a water distribution (the above described water content distribution and residual water distribution) within the plane of the membrane electrode assembly leads to local degradation of the membrane electrode assembly. In addition, a nonuniform temperature distribution within the plane of the membrane electrode assembly also causes a nonuniform power generation distribution to thereby lead to local degradation of the membrane electrode assembly. Then, local degradation of the membrane electrode assembly leads to early degradation of the fuel cell as a whole.
Then, various techniques for uniformizing a power generation distribution within the plane of the membrane electrode assembly have been suggested. For example, Japanese Patent Application Publication No. 2007-317553 (JP-A-2007-317553) describes a technique for a fuel cell system, in which temperature measuring means and cell voltage measuring means are provided at least two positions along a direction in which oxidant gas flows within a power generation plane of a cell (fuel cell), a nonuniform power generation distribution within the power generation plane is estimated on the basis of a temperature difference measured by the temperature measuring means and a voltage difference measured by the cell voltage measuring means and then the amount of coolant or oxidant gas supplied to the fuel cell is increased as the nonuniform power generation distribution increases. Then, JP-A-2007-317553 describes that, with the above technique, it is possible to reduce the influence of a temperature increase due to local current concentration within the power generation plane resulting from a significant nonuniform power generation distribution.
However, in the technique described in JP-A-2007-317553, local power generation concentration within the plane of the membrane electrode assembly due to a water distribution as described above is not considered. In addition, in the technique described in JP-A-2007-317553, the temperature measuring means and the cell voltage detecting means are provided within the power generation plane of the cell, so there is a problem that the configuration of the fuel cell is complex or the temperature measuring means and the cell voltage detecting means interfere with gas flowing within the power generation plane.